1. Field of the Invention
The invention relates to a portable navigation device used to make a dead reckoning determination, and more particularly, to a portable navigation device utilizing an audio input to determine a vehicle's speed and thereby make a dead reckoning determination.
2. Description of the Prior Art
Global Positioning System (GPS) based navigation devices are well known and are widely employed as in-car navigation devices. Common functions of a navigation device include providing a map database for generating navigation instructions that are then shown on a display of the navigation device. These navigation devices are often mounted on or in the dashboard of a vehicle using a suction mount or other mounting means.
The term “navigation device” refers to a device that enables a user to navigate to a pre-defined destination. The device may have an internal system for receiving location data, such as a GPS receiver, or may merely be connectable to a receiver that can receive location data. The device may compute a route itself, or communicate with a remote server that computes the route and provides navigation information to the device, or a hybrid device in which the device itself and a remote server both play a role in the route computation process. Portable GPS navigation devices are not permanently integrated into a vehicle but instead are devices that can readily be mounted in or otherwise used inside a vehicle. Generally (but not necessarily), they are fully self-contained—i.e. include an internal GPS antenna, navigation software and maps and can hence plot and display a route to be taken.
Dead reckoning is the process of estimating one's current position based upon a previously determined position, and advancing that position based upon known speed, elapsed time, and course. With respect to GPS navigation devices, dead reckoning allows continued navigation in the event of a loss of GPS satellite reception, or when navigating in conditions that make GPS-based positioning unreliable. Loss of GPS satellite reception occurs when driving through areas with no clear view of the sky, such as when in tunnels. GPS receivers an also produce erroneous position information when navigating through dense “urban canyons” surrounded by tall buildings, such as in large cities.
The elapsed time since a last reliable GPS signal was received is easy to measure, and an expected course of the GPS navigation device can be assumed based on a calculated route to a destination programmed by the user. Estimating the vehicle's speed, on the other hand, is not as easy to determine when GPS signals cannot be received. In contrast, when GPS signals are being received, the GPS signals will provide enough information such that the GPS navigation device will be able to calculate the vehicle's current speed. Traditionally, determining a vehicle's speed without relying upon received GPS signals is achieved by taking a tap off of the vehicle speed sensor wiring and feeding this through some signal conditioning to a processor which calculates the vehicle's speed.
Please refer to FIG. 1. FIG. 1 is a diagram of a portable navigation device 10 connected to a vehicle's wiring loom 20 according to the prior art. The portable navigation device 10 traditionally contains a display 12 for displaying map information and has a plurality of control buttons 14 for enabling a user to operate the portable navigation device 10. As stated above, in order to provide information about the vehicle's speed to the portable navigation device 10, the portable navigation device 10 must be connected to wiring in the vehicle's wiring loom 20. First of all, a protection/isolation circuit 26 is connected to both the vehicle power 22 and the vehicle speed sensor 24 of the vehicle's wiring loom 20. The protection/isolation circuit 26 obtains speed information from the vehicle speed sensor 24 and provides this speed information to a signal conditioning circuit 28 which conditions the speed signals for use in the portable navigation device 10.
Unfortunately, the prior art portable navigation device 10 has several shortcomings when having to connect to the vehicle's wiring loom 20. First of all, tapping into the vehicle's wiring loom requires a level of expertise that is beyond what most consumers are comfortable with. The second problem is that consumers are fastidious regarding wires and cables to and from devices as they prefer clean and tidy wires. A third issue is related to the cost of the portable navigation device 10, and the use of the protection/isolation circuit 26, the signal conditioning circuit 28, and the connecting wires needed to read the vehicle speed add extra cost to the overall cost of the portable navigation device 10.